1. Technical Field
This invention generally relates to electric discharge machining (EDM) and, more particularly, to an electrode retention device for use with an electric discharge machine during the formation of a side bore.
2. Discussion
In general, the electric discharge machining (EDM) process is based on the principle of erosion of the metal workpiece by spark discharges. The spark is an electric discharge through the space between the two charged elements, the first being the workpiece and the second being an electrode. The workpiece and the electrode are placed within a dielectric fluid. A direct current is applied to the system which typically includes a capacitor in parallel with the spark gap between the two elements. At low voltages the dielectric fluid acts as an insulator, but as the potential difference between the electrode and the workpiece increases there is a dielectric breakdown in the fluid and a spark passes through the spark gap. The spark causes the vaporization of some of the workpiece material as well as some of the material of the electrode. Following a spark discharge, deionization of the dielectric fluid reestablishes the insulation properties and the current again drops to near zero. The capacitor is then charged again and the process begins over. This process can be repeated at a very high rate (200-500,000 cycles per second) with the metal removal rate being controlled by the current density or average current in the discharge circuit. Typically the higher the current density the rougher the surface finish but more material is removed.
At times it is desirable to EDM a bore which is generally parallel to the primary work surface of the workpiece. An example of this condition is when a connecting passage must be provided between several deep bores already machined into the workpiece. One method of producing the connecting passage is to rotate the workpiece, or the tool, and machine the passage from the outer surface of the workpiece which is generally perpendicular to the first surface of the workpiece. The tool is positioned such that the passage passes from the outer surface through the bores which are to be connected. A plug is then inserted into a portion of the connecting passage which extends from the outer surface to the first passage which is intersected thereby sealing the passage from the outer surface of the workpiece.
There are many limitations and disadvantages to this method of providing a connecting passage. First, if there are other bores between the outer surface of the workpiece and the bores which are to be connected this method can generally not be used or if used requires the insertion of multiple plugs. Second, the use of plugs creates areas where leaks can form, particularly if high pressure air or fluid is to be used. As such, there is a need within the industry for a simple, cost effective method and apparatus to create passages which extend from within a cavity or bore while not creating unwanted passages to the outside surface of a workpiece.